netdev - Re: [PATCH net] sch_htb: fix crash on init failure

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Date:   Tue, 29 Aug 2017 21:19:05 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, jhs@...atatu.com,
        xiyou.wangcong@...il.com, jiri@...nulli.us,
        roopa@...ulusnetworks.com
Subject: Re: [PATCH net] sch_htb: fix crash on init failure

On 29/08/17 21:09, Eric Dumazet wrote:
> On Tue, 2017-08-29 at 20:58 +0300, Nikolay Aleksandrov wrote:
>> The commit below added a call to the ->destroy() callback for all qdiscs
>> which failed in their ->init(), but some were not prepared for such
>> change and can't handle partially initialized qdisc. HTB is one of them
>> and if any error occurs before the qdisc watchdog timer and qdisc work are
>> initialized then we can hit either a null ptr deref (timer->base) when
>> canceling in ->destroy or lockdep error info about trying to register
>> a non-static key and a stack dump. So to fix these two move the watchdog
>> timer and workqueue init before anything that can err out.
>> To reproduce userspace needs to send broken htb qdisc create request,
>> tested with a modified tc (q_htb.c).
> 
>> Note that probably this bug goes further back because the default qdisc
>> handling always calls ->destroy on init failure too.
>>
>> Fixes: 87b60cfacf9f ("net_sched: fix error recovery at qdisc creation")
>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>> ---
>> Always calling qdisc destroy on init failure in the default qdisc handling
>> was added in commit 0fbbeb1ba43b. I'm not sure if I should include that
>> one as fixes tag.
> 
> Well, we probably need to audit init/destroy not only in net/sched, but
> other parts of networking stack.
> 

I'm not sure I follow, I hit this while working on a net/sched/ patch and had to error
out in the init() function.

> What about the qdisc_skb_head_init(&q->direct_queue) call ?
> 
> I am surprised you do not crash in __skb_queue_purge(&q->direct_queue);
> 

Hm, do you mean in __qdisc_reset_queue() ? 
I have only tried/seen the crash happen on qdisc add.

A much simpler and easier bug is sch_multiq (ethX non-multiq):
$ tc qdisc add dev ethX root multiq
(error EOPNOTSUPP + double free of ->queues due to free in init())
and e.g. $ ip l add dumdum type dummy
to see a crash due to the corrupted memory.